Exhaust emissions from an engine, such as in a vehicle, are treated in a catalytic converter which includes catalytic material dispersed on a support. In order to treat vehicle exhaust gas emissions, the support typically includes a metal or ceramic base, which is coated with a refractory oxide or ceramic material, such as alumina. The catalytic material carried by the support promotes oxidation of unburned or partially oxidized hydrocarbons and carbon monoxide and promotes the reduction of nitrogen oxides. In the case of diesel engines, it is necessary to treat diesel particulates which are typically on the order of 0.1 micron in diameter and contain a solid, carbonaceous portion and a soluble organic portion.
Diesel particulate can be collected and removed from an exhaust gas stream by various types of filters or traps. In order to remove collected particulate, the traps are heated to an elevated temperature, which is greater than the exhaust temperature, by heaters. This approach to treating particulate is technically difficult and costly, and introduces a variety of other undesirable problems.
It is desirable to treat diesel particulate by continuous catalytic combustion of particulate in the exhaust gas stream over a catalyst in a catalytic converter in a method similar to the treatment of gaseous hydrocarbon emissions. Converters using substrates coated with alumina, such as alumina-coated monoliths, have been evaluated for this purpose. However, such alumina-coated supports promote an undesirable reaction with sulfur which is present in a relatively large amount in diesel fuels as compared to gasoline.
Silicas have been used as catalyst supports but have not been favored for use with vehicles due to the high temperature of operation of the vehicle, the relatively low thermal stability of silica as compared to alumina, and the relatively low adherence of silica to a base substrate.
In order to improve adherence of silica, U.S. Pat. No. 5,114,901 issued May 19, 1992, assigned to the assignee of the present application, describes a support consisting of two groups of silica particles of non-overlapping sizes where one group occupies voids between the other group when applied as a washcoat layer (catalyst support). Catalytic metal is dispersed on the outer surface of the applied washcoat layer (post-impregnation). Although the washcoat layer of U.S. Pat. No. 5,114,901 provides improved adherence and reduced reaction with sulfur, further improvement is desirable.